This invention relates to determining SIR (Signal to Impairment power Ratio) in a communications system. Impairments include both noise and interference.
In a communications system, such as a cellular wireless system, it is known that the SIR of a received signal is a useful signal quality parameter for various procedures. In the case of a system using two or more antennas providing diversity path signals which are subject to fading, the SIR of each path is a particularly useful parameter for use in selection or combining of the diversity path signals for optimum communications. The SIR may also be used in connection with processes such as hand-off, adaptive channel allocation, dynamic power control, and cell tiering.
It is desirable, especially for use for diversity path signal selection or combining, to provide a rapid and accurate determination of SIR for each diversity path signal. The requirements for speed and accuracy conflict with one another, in that typically averaging processes have been used to enhance the accuracy of SIR determination, which slows the determination process. Ideally, a very accurate determination of SIR would be provided instantaneously for each sample of the received signal.
It is also desirable to determine SIR in a receiver having only a single antenna, to determine SIR without reliance on any training sequence or pilot signal, and to achieve this in a manner that is reliable, simple, and easy to implement.
In the case of communications with mobile communications devices or terminals, it can also be important to determine the relative speed at which a terminal is moving. This speed determines the Doppler bandwidth of signals received from the terminal, and affects communications parameters such as fading, and hence SIR, and time requirements for hand-off. Consequently, a determination of the Doppler bandwidth of a received signal also provides useful information.
Objects of this invention are to provide improved methods of and apparatus for determining SIR and Doppler bandwidth of a received signal in a communications system.
One aspect of this invention provides method of determining SIR (signal to impairment power ratio) of a received signal in a communications system, comprising the steps of: performing a first non-linear operation on the received signal to produce a first non-linearly processed signal; separating DC and non-DC components of the first non-linearly processed signal; performing a second non-linear operation on the separated non-DC component of the first non-linearly processed signal to produce a second non-linearly processed signal; separating a DC component of the second non-linearly processed signal; and deriving the SIR of the received signal from a combination of the separated DC components of the first and second non-linearly processed signals.
The received signal can be a sampled signal and the steps can be performed for individual samples of the received signal, so that the SIR can be determined on a sample by sample basis. If the received signal is represented by a complex signal, then the step of performing the first non-linear operation can comprise multiplying the complex signal by its complex conjugate, which corresponds to a squaring operation. The step of performing the second non-linear operation can likewise comprise squaring the separated non-DC component of the first non-linearly processed signal. Although these non-linear operations comprise squaring, other non-linear operations may alternatively be performed to achieve similar results.
Preferably the step of deriving the SIR of the received signal from a combination of the separated DC components of the first and second non-linearly processed signals comprises the steps of squaring the separated DC component of the first non-linearly processed signal and subtracting from the result the separated DC component of the second non-linearly processed signal, and dividing the result by the separated DC component of the second non-linearly processed signal to derive the SIR.
Each of the steps of separating DC components of the first and second non-linearly processed signals can comprise filtering the non-linearly processed signal to produce a non-DC component of the non-linearly processed signal, and subtracting the non-DC component from the non-linearly processed signal to produce the separated DC component of the non-linearly processed signal. The method can further comprise the step of changing a parameter of the filtering in dependence upon a Doppler bandwidth of the received signal.
The method can further comprise the step of determining a power of a desired signal component of the received signal from the separated DC components of the first and second non-linearly processed signals. It can also comprise the step of determining the Doppler bandwidth of the received signal from the determined power of the desired signal component of the received signal. This preferably comprises auto-correlating the determined power of the desired signal component of the received signal, normalizing auto-correlation results, and determining the Doppler bandwidth from normalized auto-correlation results above a threshold, for example by summing said results above the threshold and/or by determining a difference of said results above the threshold.
Another aspect of the invention provides a communications system receiver including apparatus for determining SIR (signal to impairment power ratio) of a received signal, the apparatus comprising: means for applying a first non-linear function to the received signal and separating the result into first DC and non-DC components; means for applying a second non-linear function to the non-DC components and deriving from the result a second DC component; and means for combining the first and second DC components to produce the SIR of the received signal.
The means for applying at least one of the first and second non-linear functions can comprise a squaring function, and the means for combining the first and second DC components to produce the SIR of the received signal can comprise means for squaring the first DC component, subtracting the second DC component from the squared result, and dividing the result of the subtraction by twice the second DC component to produce the SIR. The receiver can further comprise means for determining a signal power of a desired signal component of the received signal from the first and second DC components, and an auto-correlator responsive to the determined signal power and means for determining a Doppler bandwidth of the received signal from normalized auto-correlation function results which exceed a threshold. All of these means can conveniently be constituted by functions of a digital signal processor.
Another aspect of the invention provides a method of determining a Doppler bandwidth of a received signal from a determined power of the signal, comprising the steps of: performing an auto-correlation of the determined power of the signal for a plurality of correlation delays; and determining the Doppler bandwidth from normalized results of the auto-correlation which exceed a threshold.
The invention also extends to apparatus for determining a Doppler bandwidth of a received signal from a determined power of the signal, comprising: means for performing an auto-correlation of the determined power of the signal for a plurality of correlation delays; and means for determining the Doppler bandwidth from normalized results of the auto-correlation which exceed a threshold.